The invention relates to a filter apparatus for fluids to be cleaned, in particular for thermoplastic synthetic plastics material for extrusion purposes, comprising a housing having a guide for at least one screen carrier member carrying at least one screen nest and being reciprocable in the guide between an operating position, a washing position and a screen exchange position, wherein in the operating position fluid to be cleaned can be supplied to the upstream side of the screen nest via at least one supply channel and at least one discharge channel is provided on the downstream side of the screen nest for the discharge of the cleaned fluid after its passage through the screen nest, whereas in the washing position the supply channel is closed by the screen carrier member, and at least one washing channel is provided for the discharge of fluid streaming through the screen nest in a direction opposite to that of the operating position and of the impurities loosened thereby from the screen nest, and wherein only in the screen exchange position the screen carrier member protrudes so far from the guide that the screen nest is accessible for the screen exchange process.
Such filter apparatus are known (for example EP-B 250 895). Within these known apparatus two screen carrier members are disposed parallel to each other in order to avoid an interruption of the supply of the extruder or the like connected to the filter apparatus during washing the screen nests of the one screen carrier member. Within this, the cleaned melt used for washing purposes is taken from the downstream side of that screen carrier member that is just in the operating position. Within this, the washing effect (cleaning effect) depends substantially from the velocity of flow of the backwashing flow that on its parts depends from the backpressure of the tool (for example the extruder head) connected to the downstream side of the filter apparatus. This backpressure, as a rule, is relatively low and cannot be influenced at will. This results in a bad freewashing of the soiled screens or--in particular at bigger screen areas --in an only partial cleaning of the soiled screens. Further, there results a loss in the mass of the main flow at the backwashing moment, what loss is as greater as longer the duration is that is necessary for a sufficient washing of the screen nest or, respectively, of the screen nests of the sieve carrier member that is just in the washing position.
It has also already proposed to dispose a rotary disc valve between two stationary screen carrier members, which valve serves for switching over between normal operation and washing operation. During the washing operation cleaned fluid is pressed from the downstream side through the screen by means of a piston that enters an enlargement of the discharge channel. Since with this the discharge channel leading to the extruder remains open, the fluid can escape through this discharge channel so that only a comparatively low backpressure is at disposal for the backwashing process. The aforesaid disadvantages, therefore, can hardly be avoided also by this construction.
The invention has at its object to avoid these disadvantages, however to retain the initially described construction and to substantially improve the backwashing effect, but at same time to avoid the loss of the mass in the main flow at the backwashing moment and to avoid an access of invironmental air to the screen nests of all screen carrier members in the washing position as well as in the operating position. The invention solves this task by the features that a storage space or a channel leading to such a space branches off in the screen carrier member from the downstream side of the screen nest, which is formed by a filter layer supported between two perforated plates, which storage space takes up cleaned fluid for the washing process, at least one piston that enters into the storage space is provided for pressing the fluid during the washing process in backwashing direction through the screen nest, also the discharge channel being closed in the washing position of the screen carrier member by the screen carrier member. This storage space can take up the amount of cleaned fluid required for the washing process at any time, however preferably only immediately before the washing process in order to avoid a detrimental thermic influence upon the fluid, for example a cracking. In order to provide for this amount of fluid serving for washing purposes in the storage space, it is only necessary to retract the piston in the storage space and thereby to suck-in fluid into the storage space until it is sufficiently filled. This can be done as long as the filter to be cleaned is in the operating position, so that the storage space is filled with fluid that is supplied from the downstream side of the filter to be cleaned. Therefore, the amount of fluid required for the washing process must no more be taken from the flow of cleaned fluid of the screen carrier member disposed in parallel, so that at the backwashing moment no additional loss of mass occurs in the main flow. Also the loss of mass when filling the storage space is neglectibly low if the retraction of the piston in the storage space is made correspondingly slowly, what can be done without any problem. Overall, however, the fact that the fluid volume provided in the storage space can be pressurized at choice during the washing process by means of the piston due to the closure of the discharge channel by the previous shift of the screen carrier member into the washing position, enables one to select the velocity of flow of the backwashing flow by means of the backwashing piston velocity, or, respectively, by means of the forces acting on the piston, at choice and--what is of still greater importance--substantially higher than this was possible heretofore. In comparison to known constructions, this results in a substantially improved cleaning effect.
The invention is not limited to the provision of a second screen carrier member that remains in the operating position during the backwashing process and from the downstream side of which the fluid volume required for the backwashing process at the first screen carrier member is branched off. To the contrary, the invention is also applicable to filter apparatus having only one single screen carrier member, if at the backwashing process a short operation interruption of the apparatus connected to the filter apparatus, for example of the extruder, can be accepted. By the feature, that the washing process at the inventive apparatus can be carried out at a high pressure and, therefore, very quickly and effectively, the time during which the supply to the apparatus connected to the filter apparatus is interrupted, is very short, so that no real operation interruption may arise there. However, an embodiment is preferred in which two or more screen carrier members are disposed in parallel to each other--with respect to the fluid flow.
Of substantial advantage is further, that the construction of the reciprocable screen carrier member shown by the initially mentioned standard of art can be maintained. This has the advantage that the screen nests of all screen carrier members are always embraced from the casing in the washing position as well as in the operation position, and, therefore, are protected against access of environmental air. This is of importance when filtering fluids that are sensitive to air, for example several kinds of synthetic plastics material that are adversely affected by the access of air in a hot condition. Only for reaching the screen exchange position, the screen carrier member is shifted so far in its guide that the screen nest to be exchanged is accessible. The rest volume of fluid contained therein, however, cannot be used anyway and is carried off. The fresh inserted screen nest is still free of fluid and, therefore, it is not of importance if this screen nest is in contact with the air, as long as the screen carrier member is in the screen exchange position. If necessary, the exchanged screen nest can be evacuated before it is guided into the operation position. It would be also possible to guide this screen nest into the backwashing position before its introduction in the operating position, and to expel in this backwashing position the air in the screen by fluid supplied from the storage space.
According to a preferred embodiment of the invention, the storage space is disposed in the screen carrier member as a whole. This, on the one hand, facilitates machining of the storage space, because one single member only, namely the screen carrier member must be machined for producing the storage space. On the other hand, the construction is easier to clean and more simple in its entire structure. As a rule, in the screen carrier member there is sufficient room for accommodation of the storage space, only in particular cases a neighbouring housing member may form a section or the entire storage space, and then a channel leading to the storage space must be provided in the screen carrier member. To provide the storage space entirely in the screen carrier member facilitates it also to convert existing plants in the sense of the invention, because only the screen carrier member must be correspondingly exchanged or machined later on.
There are substantially two basic possibilities for actuating the piston in the storage space in order to expel the fluid volume provided in the storage space and required for cleaning purposes: On the one hand, according to a further embodiment of the invention, an own drive means may be provided for advancing the piston, that drive means being independent from the movement of the screen carrier piston. This enables a completely independent movement of screen carrier piston on the one hand and the piston disposed in the storage space on the other hand, however, as a rule, these two movements will be coordinated to each other, for example so that at first the screen carrier piston is brought into the position provided for the washing process and then the piston positioned in the storage space is advanced for carrying out the real washing process. Within this, it is recommended within the invention, if the drive means, in particular a double-acting pressure medium cylinder is fixed to the screen carrier piston, in particular at the front end thereof. The drive means for the piston positioned in the storage space then moves together with the screen carrier member.
Within the spirit of the invention, the other variant consists in that the piston provided in the storage space engages with a stop of its piston rod a counter stop, preferably a bridge, fixed to the housing, preferably in a detachable manner, and does not change its relative position with respect to the housing. Within this, the backwashing process is carried out only by shifting the screen carrier piston and an additional drive means for movement of the piston positioned in the storage space and fixed to the screen carrier piston can be saved. This results in a more simple and more economic construction.
As a rule, within the spirit of the invention the storage space is formed by an axial bore of the screen carrier channel,that bore extending up to the one front end of the screen carrier member. Within this, according to a preferred embodiment of the invention,the construction may be so chosen that the front surface of the piston facing the screen nest in the piston position nearest the screen nest is flush with the wall of the space disposed behind the screen nest in the screen carrier member, when seen in normal flowing direction of the fluid. The backwashing piston is then flush with respect to the main flow channel on the clean side of the screen nest (filter carrier). In operation of such an apparatus it is to be recommended to fill the backwashing reservoir only immediately before the real backwashing process. This avoids dead corners in which stillstanding hot fluid masses remain for a longer time. This is of importance in particular at synthetic plastics melts that tend to a thermic decomposition when being subject to a longer intense heating.
According to an advantageous further embodiment of the invention, the drive means for the piston positioned in the storage space comprises control means for influencing the movement of the this piston, for example a throttle check valve inserted into a supply line for the pressurized medium for the pressure cylinder serving for movement of the backwashing piston. This enables one to adjust at choice the quantity of the melt branched off from the main melt flow when filling the backwashing reservoir, without that for this a throttle means, for example a throttle nozzle, is necessary that is inserted into the backwashing channel and controls the backwashing flow. It is also of advantage that the control means used within the invention is disposed on the outside and therefore is always accessible for adjustment, service or exchange.